The present invention relates to the protecting of power output stages and more particularly to the protection of transistor output circuits which are normally employed for driving reversible motors or other loads needing high initial current.
In transistor power output stages for driving reversible motors, there are typically provided four output transistors which are connected in an H pattern so that, by turning on diagonal pairs, current may be applied through the motor in either direction. To prevent destruction of the output transistors if there is a short in the load or if either output terminal is accidentally shorted to the opposite power rail, the output transistors are typically operated in either a constant current mode or the driver circuits employ current sensing so that the total power dissipation in the output device is limited to a safe level taking into consideration of the packaging used to house the circuitry and the thermal resistance of the package.
A conflicting need is the requirement of supplying substantially higher current during starting of the motor than is required during running. It is advantageous to be able to provide a current up to an order of magnitude greater than the normal running current to obtain fast startup. Such current levels are far in excess of those which could be tolerated during a short circuit. Accordingly, it has heretofore been necessary to sacrifice either complete short circuit protection or the availability of high startup current.
Among the several objects of the present invention may be noted the provision of novel transistor power output stages; the provision of such power output stages which are suitable for driving reversible electric motors the provision of such output stages which will provide high levels of startup current to an electric motor; the provision of such output stages which afford short circuit protection with respect to the output terminals connected to the motor; the provision of such output stages which may be implemented in an integrated circuit; the provision of such output stages which are highly reliable and which are of relatively simple and inexpensive construction. Other objects and features will be in part apparent and in part pointed out hereinafter.